Campbell Penetration Depth of a Superconductor in the Critical State

Abstract

The magnetic penetration depth $\lambda (T,H,j)$ was measured in the presence of a slowly relaxing supercurrent, $j$. In single crystal $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_8$ below approximately 25 K, $\lambda (T,H,j)$ is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. The Campbell length depends upon the ratio $j/j_c$ where $j_c$ is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.